Passivation of dichromated gelatin holograms to high relative humidity ambients

ABSTRACT

A method of passivating dichromated gelatin holograms to high relative humidity ambients is disclosed. The method involves a developing process whereby a dichromated gelatin hologram, which has been exposed to a laser light, is initially treated with a first solvent which both swells and photographically fixes the exposed hologram and also introduces a high relative humidity passivating species into the photographically fixed holographic film. The swollen and fixed hologram is then treated with a second solvent which both removes the first solvent and develops the hologram. A subsequent baking of the developed hologram passivates it to high relative humidity conditions.

ggsc i wol U uncu Dlll Shankoff et al.

[54] PASSIVATION OF DICHROMATED GELATIN HOLOGRAMS TO HIGH RELATIVEHUMIDITY AMBIENTS [72] Inventors: Theodore Arthur Shankoff, Mendham;Constance Veronica Stanionis, Piscataway, both of NJ.

21 Appl.No.: 28,136

[52] US. Cl. ..96/49, 96/27 H, 96/93, 350/35 [51] Int. Cl. ..G03c 5/22,G03c 5/04 [58] Field ofSearch ..96/27 l-l,49,93,75; 350/35 [56]References Cited UNITED STATES PATENTS 3,023,099 2/1962 Yaeger et al..96/93 X 2,199,865 5/1940 Wood 96/93 X 2,675,315 4/1954 Staehle et al..96/93 X 3.012.886 12/1961 Lerner 96/93 X 2,359,040 9/1944 Jorgensen96/93 X 2,484,431 10/1949 Staehle et al 96/93 X 1.609091 11/1926 Miller96/93 X 2,652,345 9/1953 Jones 96/114 X 2,716,060 8/1955 Lupo ..96/93[451 May 2,1972

3,567,444 3/1971 Shankoff ..96/27H OTHER PUBLICATIONS Lin, L. H., ApliedOptics, Vol. 8 No. 5, 5/1969, pp. 963- 966.

Brandes, R. G. et al., Applied Optics," Vol. 8 No. 11, ll/l969, pp. 23462348.

Kosar, J., Light-Sensitive Systems, 1965, Wiley & Sons, pp. 46 48, 52-62, 67, 68, 78- 80, 82- 90.

Shankoff, T. A., Applied Optics, Vol. 7, No. 10, 10/1968, pp. 2101-2105.

Primary ExaminerCharles L. Bowers, Jr. Attorney-R. J. Guenther and EdwinB. Cave [5 7] ABSTRACT A method of passivating dichromated gelatinholograms to high relative humidity ambients is disclosed. The methodinvolves a developing process whereby a dichromated gelatin hologram,which has been exposed to a laser light, is initially treated with afirst solvent which both swells and photographically fixes the exposedhologram and also introduces a high relative humidity passivatingspecies into the photographically fixed holographic film. The swollenand fixed hologram is then treated with a second solvent which bothremoves the first solvent and develops the hologram. A subsequent bakingof the developed hologram passivates it to high relative humidityconditions.

3 Claims, No Drawings OR to es/. 9

PASSIVATION F DICHROMATED GELATIN HOLOGRAMS TO HIGH RELATIVE HUMIDITYAMBIENTS BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to a method for the passivation of dichromated gelatinholograms to high relative humidity ambients and more particularly, tothe passivation of a hologram in situ during the development of theexposed hologram.

2. Description of the Prior Art The gelatin-dichromate photosensitivesystem has been shown to be very efficient as a recording medium in thefield of holography. This efficiency stems from the fact that inholography, reconstruction can be from a recording of either varyingoptical density, or from one of varying refractive index. In the lattercase, a hologram can be theoretically 100 percent efficient, that is, beable to diffract all of the reading laser light into the reconstructedimage. This refractive index or phase hologram is approximately 20 timesmore efficient than the absorption hologram recorded on conventionalfilm. Dichromated gelatin phase holograms commonly exceed 90 percentefficiency for the simplest type of hologram, i.e., the plane grating.Efficiencies for other types of holograms are generally to times greaterthan those in photographic films. Besides being a valuable material inthe field of holography, dichromated gelatin has been shown to provideholographic plane gratings of almost theoretical spectroscopic resolvingpower.

An important limitation on the use of dichromated gelatin as a phasehologram material has been the inability of developed holograms toresist high relative humidity ambients. There is a tendency for thedeveloped hologram to degrade under such conditions in a relativelyshort time. In this regard, the tendency to degrade increases withincreasing film thickness. Attempts to passivate these holograms byovercoating or encapsulating the hologram with polymeric films have notbeen entirely successful. The polymer encapsulation has been found to beeffective only against exposure to liquid water. Owing to the highdiffusion coefficients for water vapor through all known polymers, theencapsulation has been only partially successful in the protection ofthe hologram from water vapor and only under conditions where thepolymer film has been relatively thick. The thickness of the polymerfilm has led to the additional problem of optical distortion. Aprocedure for the stabilization of developed holograms against highrelative humidity conditions has, therefore, long been sought.

SUMMARY OF THE INVENTION The present invention is directed to a methodfor the passivation of dichromated gelatin holograms to high relativehumidity ambients. The method involves a developing process wherein anexposed hologram is initially treated with an ammonia neutralizedphotographic fixing solution containing CrO- the solution beingtheorized to partially develop the exposed hologram. It is alsotheorized that an ammonium-linked chromium oxide complex is incorporatedin the photographically fixed hologram. The chromium trioxide isthermally activated after the permanent development of the hologram andreacts with the hydrophillic groups responsible for moisture absorptionin the gelatin film, thus passivating the recorded holograms to highrelative humidities.

DETAILED DESCRIPTION The present invention is based upon a method ofpassivating dichromated gelatin holograms to high relative humidityambients wherein the bulk of the hydrophilic groups on the gelatinmolecule are tied up. However, since the mechanism of hologramdevelopment requires that these hydrophilic groups be active throughoutthe development procedure, the method employs a Cr ion containingspecies which does not affect the gelatin during development but isincorporated into the film and activated at a later time as CrO Agelatin-coated glass plate or slide is prepared by applying an aqueousgelatin suspension containing a small amount of (NI-l Cr O to thesurfaces thereof, employing any standard means such as, for example,spinning, spraying, doctor blading or dipping. The suspension-coatedplate is air dried and then baked in an oven to form a hardened gelatincoating. The gelatin-coated glass substrate can then be stored for aperiod of at least 6 months prior to sensitization.

The gelatin-coated glass plate is subsequently sensitized by dipping thegelatin coat or film into an aqueous solution of (NHQ Cr O for severalminutes. In this regard, it is to be noted that a small amount of awetting agent or surfactant may be added to the aqueous (NHO: CF20?solution in order to ob tain a uniform sensitization or dichromateconcentration throughout the entire gelatin film thickness.

After applying the aqueous (NI-I C O the dichromated gelatin plates areallowed to dry either in darkness or under a red light. Upon drying, thefilm is stored at a maximum temperature of 20C in darkness and should beused within 12 hours. The sensitized plates are then exposed to laserlight to form holograms or diffraction gratings. It is important to notethat the hologram at this stage was formerly treated with a solutionwhich washed out any Cr ion containing species contained therein andarising from the sensitizer.

A solution to photographically fix the exposed hologram, to swell thegelatin, and to incorporate the Cr ion containing species into thephotographically fixed hologram is prepared by dissolving an amount ofCrO in a suitable first solvent which is water. To the first solvent maybe added a suitable wetting agent or surfactant to insure uniformswelling and fixation over the entire exposed hologram film surface.

The first solvent is maintained at a temperature in the range of 20-40C, and 0.5-5.0 weight percent CrO is added to form an acidic solution.NI-I OI-I is then added to neutralize the resultant acidic solution topH=7. It is postulated that the ammonia neutralization of the acidicsolution containing CrO leads to the formation of an ammonium-linkedchromium oxide species which is incorporated into the photographicallyfixed hologram film prior to the permanent development thereof. Theneutralization of the acidic solution with NI-I alleviates problemsconcerning photographic speed and noise in the holographicreconstruction. It can also be hypothesized that the neutralizationchecks oxidation of gelatin by CrO during development and also preventsthe acid hydrolysis of gelatin.

The exposed hologram is immersed in the neutral solution, maintained ata temperature in the range of 20-40 C, for a period of time sufficientto effect the ends designated (1) and (2) above and to (3) incorporatethe Cr ion containing species, hypothesized as an ammonium-linkedchromium oxide species, into the photographically fixed hologram. It isalso theorized that the neutral solution partially develops thehologram. A suitable second solvent is then selected which is anonsolvent for gelatin but which is capable of l) rapidly dissolving thefirst solvent, (2) shrinking the swelled photographically fixed hologramto approximately its initial size, and (3) permanently developing thephotographically fixed hologram. Suitable second solvents are acetone,methylethyl ketone, ethanol, 2-propanol, dioxane, isopropanol, ormixtures thereof.

A bath of the second solvent is maintained at a temperature ranging fromroom temperature to the boiling point of the second solvent and thehologram is immersed therein for a period of time sufficient to removethe neutral solution from the surfaces of the fixed hologram, shrink thehologram to approximately its original size and to develop thephotographh cally fixed hologram. The hologram is thereupon removedtherefrom, air dried and baked at ca. C for a period of time sufficientto harden the developed film. During this heat treatment of the hologramit is hypothesized that there is an evolution of NI-I gas resulting inthe reformation of CrO which species passivates the developed hologramto high relative humidity ambients.

The hologram resulting from the above-described treatment is nowpassivated or stabilized to high relative humidity conditions. Thestability level is taken as that relative humidity in which planegratings with 0.5;]. d-spacings show no loss in diffraction efficiencyafter a 72-hour exposure. If desired, in order to protect the hologramfrom liquid water, a thin coating of a suitable polymeric material maybe applied to the stabilized hologram. A suitable polymer may beselected from vinyl polymers such alkyd resins, acrylic polymers,methacrylic polymers, polyvinyl chlorides or polyvinylidene chloride.However, it is to be noted that the thickness of this film should bekept to a minimum to avoid any possibility of optical distortion.

A further understanding of the method of this invention will be obtainedfrom the following specific examples which are illustrative of theinvention and are not restrictive .thereof.

EXAMPLE 1 A sufficient amount of commercially available USP gradegelatin having a bloom strength of 125 was weighed out and added towater to form a 16 percent by weight aqueous gelatin suspension at 40 C.In this regard, bloom strength equals the weight required to force astandardized cylindrical plummet 4 mm into the surface of a gelatin gelmade of 6.67 percent aqueous gelatin which is maintained at 10 C for 17hours. To this suspension was added (NI-I Cr O which was of the highestpurity commercially available. The amount of (NI-l Cr O, added wassufficient to give a weight ratio of 0.5 percent to the weight ofgelatin.

A X 5 X 0.058 in. to 0.068 in. glass plate, commercially obtained havingan index of refraction of 1.523 and a window glass finish, was dippedinto the resultant dichromate-gelatin suspension employing a commercialdip-coating apparatus. The plate was withdrawn from the suspension aftera time period sufiicient to form a 311. thick dichromate-gelatin coatedplate, after subsequent drying and baking operations. Thedichromate-gelatin coated plate was then set in a vertical position toair dry for 1 hour. The dichromate-gelatin film-coated plate was thenhardened in an oven at 150 C for 2 hours. The 3p. thick hardenedfilm-coated plate was then sensitized by being dipped vertically in a4.0 percent aqueous (NH Cr O, bath maintained at 40 C and containing20,000 parts by weight of a wetting agent. After 2 minutes thesensitized plate was removed and vertically dried in the dark.

The dried sensitized plate was exposed to 5 mjCm of 4880 A. laser lightto form a gelatin hologram. The exposed plate or hologram was thenimmersed in a fixing bath neutralized with Nl-l Ol-l to a pH=7 andmaintained at 25 C. The fixing bath contained a first solvent of waterand 0.005 percent by weight of a commercially available surfactant. Tothe first solvent was added 0.5 percent by weight CrO of the highestpurity commercially available, to form an acidic solution. NH OH wasimmediately thereafter added to neutralize the acidic solution to a pHequal to 7.

After 1 minute the photographically fixed hologram was removed from theneutralized fixing solution and immersed in a bath maintained at 70 Cand containing a second solvent of isopropanol. The fixed hologram waskept in the isopropanol for 30 seconds in order to remove the firstliquid and to shrink EXAMPLE 2 The procedure of Example 1 was repeatedwith the exception that 1.0 percent, by weight, CrO was added to thefirst solvent. The resultant hologram was stable up to a relativehumidity of 84 percent at 25 C.

EXAMPLE 3 The procedure of Example 1 was repeated with the exceptionthat 2.0 percent, by weight, CrO was added to the first solvent. Theresultant hologram was stable up to a relative humidity of 93 percent at25 C.

EXAMPLE 4 The procedure of Example 1 was repeated with the exceptionthat 5.0 percent, by weight, CrO was added to the first solvent. Theresultant hologram was stable above a relative humidity of 95 percent at25 C.

EXAMPLE 5 The procedure of Example 1 was repeated with the exceptionthat a 10p. thick dichromate-gelatin coated plate was formed. Theresultant hologram was stable up to a relative humidity of 73 percent at25 C.

EXAMPLE 6 The procedure of Example 5 was repeated with the exceptionthat 1.0 percent, by weight, CrO was added to the first solvent. Theresultant hologram was stable up to a relative humidity of 84 percent at25 C.

What is claimed is:

l. A method of passivating an exposed material to high relative humidityambients, said exposed material having a dichromated gelatin coatingcomprising gelatin and a dichromate compound and upon development formsa phase hologram in the exposed material, said method comprises:

a. immersing the exposed material in an NH OH neutralized first solventsolution having a pH of 7 comprising water having CrO dissolved therein,thereby photographically fixing the exposed material;

b. removing the material from said first solution and immersing it in asecond solvent, thereby developing the photographically fixed material,said second solvent being selected from the group consisting of acetone,methylethylketone, ethanol, 2-propanol, dioxane, polyglycol ethers,isopropanol and mixtures thereof;

c. removing the hologram from said second solvent and heating it at atemperature of C thereby resulting in a high relative humidity resistantdeveloped hologram.

2. The method as defined in claim 1 wherein said dissolved CrO ispresent in an amount within the range 0.5 to 5 percent by weight of CrO3. The method as defined in claim 1 wherein said first solution ismaintained at a temperature in the range of 25-40 C.

2. The method as defined in claim 1 wherein said dissolved CrO3 ispresent in an amount within the range 0.5 to 5 percent by weight ofCrO3.
 3. The method as defined in claim 1 wherein said first solution ismaintained at a temperature in the range of 25* - 40* C.